Pulse width modulated (PWM) inverter supplied permanent magnet (PM) motors can be operated and controlled as variable-speed synchronous machines. Two independent parameters, i.e. the load angle and the ratio between applied voltage and EMF, can be used for real power (torque) control and reactive power (power factor) control, respectively. The present invention is concerned with power factor control.
The parameter used for reactive or power factor control is the amplitude of a pulse width modulating signal. This signal is automatically adjusted as a function of motor speed, DC link voltage and load angle. However, power factor can vary as changes in the EMF (magnet strength) and motor resistance occur, usually due to temperature changes. Heretofore, these EMF and resistance variations were compensated for manually during the operation of the motor.
The present invention automatically adjusts the pulse width modulating signal for approximately unity power factor operation using Park Vectors or space Vectors as they are more commonly known.
It will be appreciated that traditionally power factor has been defined only in terms of sinusoidal quantities. Direct measurement of power factor requires monitoring of zero-crossovers of voltage and current waveforms. The time difference between corresponding zero-crossover instances is a measure of power factor. If voltage and/or current waveforms have a high harmonic content, heavy filtering is required for unambiguous results.
An alternate way to measure power factor related quantities is to generate the reactive component of current. Due to the harmonics generated by the pulse width modulating process, Park Vectors must be used in place of the usual sinusoidal phasor quantities. Park Vectors inherently contain information on both the instantaneous magnitudes and phase relationship of three-phase rotating fields with respect to a reference coordinate system. Therefore, in a "synchronously rotating coordinate frame" the imaginary component of the current Park Vector is a measure of reactive power and power factor.
The signals available to generate the required Park Vectors are modulating voltage (from EMF feedback) and motor current. Signals from at least two phases must be available, and motor current can be filtered or unfiltered.
To appreciate the Park Vector features of the present invention, it will first be understood that "vector diagrams" have long been used to graphically describe the performance of electrical machines. The term "vector diagram" has generally been replaced by the term "phasor diagram." As long as the voltages and currents represented in these diagrams are steady state sinusoidal quantities, there is little distinction between the terms. The primary feature of a phasor diagram is that it can display the steady-state phase difference between a voltage and its associated current. The "phase shift" between the voltage and the current in a phasor diagram is a measure of power factor.
When electrical quantities are non-sinusoidal and/or in a transient state, the phase relationship between a voltage and its associated current can vary from instant to instant and "phase shift" has no meaning at a given instant. The circumstances become even more complex in the case of three phase electrical machines, since the voltages and currents of the individual phase windings, in both the machine stator and rotor, can vary independently of each other. Thus, simple phasor diagrams are not sufficient to describe the aforementioned non-sinusoidal and/or transient phenomena. For this reason Park Vectors (also referred to as three-phase vectors or space vectors) are introduced.
Park Vectors described as aforenoted and as applied to a three phase electrical machine are described in the text "Transient Phenomena In Electrical machines" by P. K. Kovacs, published by Elsevier in 1984, the same being incorporated herein by reference. It should be noted that under steady-state conditions a Park Vector degenerates into a phasor.
The applicants herein are aware of the following prior art relating generally to the present invention: U.S. Pat. No. 4,047,083 which issued to Plunkett on Sep. 6, 1977 (U.S. Class 318/231); U.S. Pat. No. 4,186,334 which issued to Hirata on Jan. 29, 1980 (U.S. Class 318/805); U.S. Pat. No. 4,533,836 which issued to Carpenter, et al on Aug. 6, 1985 (U.S. Class 307/11); U.S. Pat. No. 4,772,996 which issued to Hanei, et al on Sep. 20, 1988 (U.S. Class 363/41); U.S. Pat. No. 4,788,485 which issued to Kawagishi, et al on Nov. 29, 1988 (U.S. Class 318/811); and U.S. Pat. No. 4,855,652 which issued to Yamashita, et al on Aug. 8, 1989 (U.S. Class 318/268).
The Plunkett '083 patent discloses a regulating and control circuit for an adjustable speed motor involving a torque regulator. The torque regulator causes a frequency command signal to differ from the actual speed feedback signal of the motor. An inverter whose power is taken from a power source supplies power to three phase stator windings of the motor.
Hirata '334 discloses a control system for an AC motor involving a power factor detector, an adder/subtractor circuit, and an inverter. The power factor detector differentiates the signal from a phase controller.
The Carpenter '836 patent discloses a multi-voltage switching power supply comprising solid state switches and pulse width modulation circuits. Circuits are operated by a fifty percent duty cycle control circuit. The peak value of the current applied to the power supply circuit is controlled by adjusting the closure time of switches.
The Hanei '996 patent discloses a load current detecting device for a pulse width modulated inverter involving a DC power source, a main circuit and a waveform command generating circuit. The circuit generates a three phase output.
The Kawagishi '485 patent discloses a circuit for controlling an AC motor involving a three phase source, a three phase converter, a pulse width modulated inverter and a current detector circuit (40). The current detector circuit detects the DC current passing through the motor.
The Yamashita '652 patent discloses a speed control device for a motor, comprising a microcomputer, an inverter driver, an inverter and a current controller. The controller controls the waveform and magnitude of the power supply current.
None of the aforementioned patents teach the particular arrangement herein disclosed as will be discerned from the description which follows.